nothing right now! NVidia ungimp tool isn't out yet so only 2 are being used. But this PC has a 4k 32" gsync Acer screen and the other PC is a 65" 4k HDR OLED TV.
I'm looking forward to DP 1.3 (or 1.4) 32"+ 4k screens with over 60hz. Maybe that Dell 30" 4k OLED will be released at some point. Nvidia has had one for testing for a few months.